Quick fix to avoid the layer index to get unusable in case of tx rollback

This PR will disable the caching of the previously added geometry node.

Background: If the transaction in which a geometry node is added to the db is rolled back, previousGeomNode will never be reset, making the whole index unusable because of a NotFoundException being thrown by 

    previousGeomNode.createRelationshipTo(geomNode, SpatialRelationshipTypes.NEXT_GEOM);

There might be better solutions, but maybe this is something to start from.